terraform: adding .tofu extension used by opentofu

[matteoredaelli]

Description

Opentofu is a fork of terraform supported by the Linux Foundation. Opentofu recommends to use .tofu extension instead of .tf

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactoring (no functional changes)
• Documentation update
• CI / Build / Tooling

How Has This Been Tested?

• make test passes locally
• Manual testing (describe below)

Checklist

• My code follows the project's coding style (go fmt, go vet)
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or my feature works
• New and existing unit tests pass locally with my changes
• I have updated the documentation accordingly (if applicable)
• I have signed the CLA
• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

Related Issues

[CLAassistant]

All committers have signed the CLA.

[github-actions]

✅ OpenCodeReview: Review complete: 0 finding(s) across 2 selected item(s).

[chaojixinren]

Looks good to me overall.

One thing I noticed from the OpenTofu docs is that it also supports .tf.json and .tofu.json configuration files. Do you think it would make sense to support those here as well, or should we leave them for a follow-up PR?

[matteoredaelli]

I prefer not to manage .tf.json and .tofu.json in this PR.. Thanks Matteo